The present invention relates to a fuel injection arrangement for multi-venturi tube (MVT) type main fuel injectors for a gas turbine combustor and particularly relates to fuel injection locations within the venturi for optimizing fuel distribution, fuel/air mixing and sensitivity to air mass flow distribution among the venturis.
A venturi is an aerodynamic device consisting of a converging inlet, a throat and a diffuser. Typically, venturis are circular in cross-section and are sometimes used in fuel injectors in combustors for certain types of gas turbines. The venturis in the combustors of these turbines precondition the flow before the fuel/air mixture flows into a catalyst inlet, provide for fuel injection and afford pre-mixing of the fuel/air mixture with minimum pressure drop. See for example U.S. Pat. Nos. 4,845,952 and 4,966,001. The uniformity of the fuel/air mixture at the catalyst inlet must be maintained over a large cross-sectional area. In prior applications, e.g., the above patents, fuel/air mixing is accomplished by distributing the fuel among a large number of venturis, e.g., over one hundred, that populate the combustor cross-section followed by aerodynamic mixing inside the venturi tubes as well as in the downstream region between the exit planes of the venture tubes and the catalyst inlet.
Because a high level of fuel/air uniformity is required at the catalyst inlet and mixing inside the venturi tubes is limited, large recirculation regions that form at the venturi exits are typically relied upon for complete mixing. However, there is a potential for flammable mixture formation in the wakes of the venturi gaps, i.e., the areas between the diffuser exit openings downstream from the venturis. This leads to potential deleterious flame-holding events. Further, in prior venturi designs, fuel injection supply holes were located at the throat of the venturi tubes where the primary fluid velocity is highest. This takes advantage of the low static pressure at the throat. However, it has been found that such fuel supply location vis-a-vis the venturi is not optimized for fuel injection and efficient mixing.
The amount of mixing that takes place inside the venturi tube is directly related to jet penetration which in turn depends on the pressure ratio across the fuel injection holes and on the jet momentum ratio (between the jet and the mainstream). The pressure ratio is very low particularly at low loads (low fuel flow) and the fuel jet is weak (jet momentum is low compared to the momentum of the main flow). Fuel supply jets located at the venturi throats are also sensitive to mass flow distribution among venturis. That is, if one venturi flows more air than another, the velocity at the throat will be higher (static pressure would be lower) in that venturi and the venturi will suction a greater magnitude of fuel. One or more fuel jets at throat locations of the venturi also upset the boundary layer and cause flow separation inside the venturi diffuser with adverse impact on flame holding resistance. Additionally, the flow separation inside the diffuser may be a result of flow disturbance caused by the wakes at the venturi exits.
Further, from the standpoint of the operational life of the catalyst, efficient and safe operation of a catalytic combustor requires the catalyst to be active and fueled over a wide range of loads. Thus, it is required to maintain optimum fuel distribution among the venturi tubes over the entire operational range of flows in order to meet the fuel/air uniformity which is critical to quality at the catalyst inlet. Consequently, there is a need for a multi-venturi tube fuel injection system for optimizing uniform fuel/air mixtures inside the venturis, improving fuel distribution among the venturis and reducing the sensitivity of fuel injection to air mass flow distribution among the venturis.